<gasdefinitions>

In this section all breathing gases used are described. (General data of tanks as parts of the equipment are given
in the <tank> section within the <equipment>
section, while specific tank data — for example pressure at begin and end of dive — are listed with
the respective dive (<dive> -> <tankdata>).)
For each breathing gas a separate <mix> subsection has to be created.
<mix> is the only element within
<gasdefinitions> and can be repeated any times needed.